Radio-proof electric firing device



Dec. 22, 1959 EL. ALFORD RADIO-PROOF ELECTRIC FIRING DEVICE Filed Sept. 50, 1957 Ill A'Tou/m/ A. ALFoAw INVENTOR ORNEY "cuitry of a guided missile be radio-proofed. successfully accomplished, an enemy is certain to'em'ploy countermeasures to prematurely explode the missile and accomplishthis result.

W 2,918,001 Patented Dec. 22, 1959 ice United States Patent f Edwin L. Alford, Hopewell, Va), assignor of one-half to William W. Gar-ber, Richmond, Ya.

Application September 30, 1957, Serial No. 687,233 Claims. (oi. 102-28 Thisinvention relates to electric initiators and more vpa'rticularly'to clectr'icfiring devices having construction to any radio frequency induced currents which may be featiires which prevent their being prematurely fired by radio frequency energy.

Electric blasting caps are fired by passing an electric 'cirent, usually direct current, through a pair of lead wires which are connected inside a small detonating cap shell by a filament or bridge wire of high electrical resistanc'e. A sufiicient flow of current heats the bridge wire toincandescence, thereby, igniting the surrounding heat-sensitive charge.

Those familiar with electric blasting caps, electric squibs and the like are well aware that these devices are subject to accidental discharge by RF energy. Electric firing techniques include procedures intended to minimize this possibility, for example, it is common practice to con'ducttests and make a radio hazard survey to determine if it is safe to undertake electric blasting operations in'a given area. However, such precautionary measures 'are'of dir i'iinishing valu'e because it is becoming increas- '"ingly difficult to predict the extent ofradio hazard from one moment to the next.

Obviously, the radio hazard increases as the radio frequency services continue to expand. Thus, it is apparent that radio-safe electric firing devices must be devised if serious accidents are to be averted.

Itis of utmost'importance that the electric initiator cir- If this is not exploit this weakness. p I i 'In'application Serial No. 670,992, filed July 10, 1957,

the hazards causing premature firing of "electric blasting caps are further explained and an initiatoris *de'crib'ed incorporating 'a'fuse which opens the circuit to the bridge wire to prevent accidental firing upon induction of too high radio frequency current inthe lead wires. While resistant initiator not only prevent accidental -firing,'but also insure purposeful firing at the time needed. 'The present invention is directed to a construction which will Having in mind the increasing need for making electric initiators radio'safe, it is a primary object of the present invention'to 'provide'means" for radio-proofing certain types of commercial and military electric 'firing devices while insuring that they will fire when desired.

Another ,bbjectof the invention is'to provideanRF fj'ener'gy blocking device which can be incorporated in the firing circuit of electric initiators which' are themselves "tooj smallfto contain the elements of the invention.

Generallydescribed, the invention is 'ar'adio-p'roof electric firing device having in combination with the elecircnitry includinga 'coil'whi'ch is designed to perform present in the input lead wires so as to prevent premature firing by said radio frequency induced currents, and second, under the flow of firing current to attract an armature in the manner of a solenoid or relay and connect the bridge wire to firethe device. The coil is so designed as to operate properly with firing current which is preferably D.C., but which maybe pulsating DC. or even AC. of certain low frequencies. For example, the device could be constructed so that no difficulties would be involved in the use of a firing current ofup to 800 c.p.s. A.C. I z

The novel structure of the present invention may be employed in or inconjunction with any type of electric initiator, such as blasting caps, electric squibs and certain military detonators and firing devices to protect against premature discharge by radio frequency energy. However, since the commercial electric blasting cap is perhaps the most commonly used initiator, it will be appropriate and convenient to illustrate/the invention as applied to such device. 7 p V The novel features that are considered characteristic of the invention are'set forth with particularity inth'e appended claims. The invention itse'lf,.however, both as to its organization and its method of operation, together with additional objects and advantages thereof, will best be understood from the following description of specific embodiments when read in connection with the accompanying drawing, wherein like reference characters indictate like parts throughout the several figures and in which:

Fig. 1 is a sectional view of an electric blasting cap incorporating the present invention;

Fig. 2 is a circuit diagram of the blasting cap shown in Fig. 1;

Fig. '3 is a sectional view of another embodiment of the invention; and a Fig. '4 is a sectional view of a modified solenoid assembly adaptedfor usewith a firing jdevice, parts of which are diagrammatically shown.

Referringin detail 'to-the drawing, Fig. 1 illustrates an electric initiator as comprising a tubular cap shell 3 preferably formedo'fplastic material and containing a compressed'fulmi'n'a'ting mixture '5 topped by 'a loose fulminate 7, the latter serving as an ignition-charge. The previously opentop ofthe cap shell is sealed by a rubber plug 9. An ignitionplugfllretains the loose fulminate 7, and input lead wires 13 and 14 extend through the rubber plug 9. The input'lead wires are connected to a coil 15 which has the designed dual function of "an RF-choke and a solenoid. The coil is wound on a 1101- low cylindrical form 17. A tap wire 19 extends from point 18 on coil 15 to a contact 21 supported at the top of the coil form. Asimilar contact is designated by numeral 23 and from this contact runs a' lead 24 to one terminal ofbridge wire 2'5,-whose other terminal is connected by wire 2'6 to input leadwire 14 connecting at 'point'1'2. Within the coil form 17'is a small spring 27 which supports an armature 29, urging the armature head 31 away from contacts '21 and 23. The armature is formed with a recess 10 in its lowerside which-pro- 'vide's space for the armature head 31' yet limits-its upward movement.

The circuit of' th'e blasting capo'f'Fig. 1 'is dia'gi'ammed in Fig. 2 and functions in-the followingmanner: 'Coil 151s designed "to offer sufficientimpedance to any energy which may be' induced in input lead Wire's 1 3a'n'd 14 to preventa solenoid action which would pull ":in armature 29. Consequently, induced R. F. energy cannot 'rea'chbridge Wire 25 to'detonate the blasting cap since the bridge wire circuit remains open at contacts 21 and 23. However, the application of D.C., or low frequency A.C.,.firing energyto input leads 13 and .14 will so energize the coil 15 as to produce a solenoid action and cause the armature head 31 to bridge contacts 21 and 23, thereby, allowing a current of detonating value to. flow through the bridge wire 25. When contacts21 and 23 are bridged, the upper portion of coil 15 is shunted out because of tap wire 19. This increases the current flow through the lower portion of the coil and holds the armature against contacts 21 and 23 until firing is accomplished. The fitting circuit may be traced from the illustrated, positive terminal of a source of D.C. energy through input lead Wire 13, one section of coil 15 (corresponding to the lower section in Fig. l), connection 18, tap wire 19, contact 21, armature head 31, contact 23, lead 24, bridge wire. 25, lead 26, connection point 12, and input lead wire 14 to the negative terminal of the source.

blasting cap of Fig. 3 differs from that of Fig. 1 in that.

coil 35 is inverted and the spaced contacts 21 and 23 are located at thebottom of tubular coil form 37,. The

'tubular coil form 37 is embedded at its upper end in a supporting disc 38, preferably of rubber or the like. The initiator is capped by a seal 39 of asphalt, sulphur or similar material.

In place of armature 29 and spring 27 shown in Fig, 1, the modified construction of Fig. 3 utilizes an armature 32 headed by a disc or bridging contact 33 at its lower end which rests on the ignition'plug 11 spaced from contacts 21 and 23. This spacing is normally maintained by a resilient plastic ring 34 which is apertured diametrically through its wall to permit sliding passage of the cylindrical body of armature 32, and which is positioned between the bottom end of coilform 37 and the armature head 33. When firing current is applied to input leads 13 and 14, the magnetic flux in coil lifts the armature 32, carrying bridging contact 33, against the pressure of ring 34. The ring 34 is not able to withstand such lifting force and is flattened somewhat, permitting the armature head 33 to make contacts 21 and 23. In other respects the blasting cap of Fig. 3 functions in the same manner as described for that of Fig. 1. t

In Fig. 4, a modification of the invention is illustrated ,which operates like those previously described. This radio-proof electric firing device may be applied within or outside an electric initiator. The coil 45 is wound in multilayers of turns on a short tubular-form 47. Again the coil 45 is wound with wireof two sizes and spaced contacts 21 and 23 are supported on the upper end of insulating form 47. The armature 49 is L-shaped and formed of resilient conducting material. One armature leg is taped, as designated by 50, cemented or otherwise secured to the outer turns of coil 45. The other leg of armature 49 normally is spaced above contacts21 and 23. When D.C. firing current flows in coil 45,

. armature 49 is attracted and distorted in such manner as to engage contacts 21 and 23 making the connection to bridge wire 25.

In firing devices according to the invention, continuity checks to test completeness of the firing circuits can be made by simply depressing the armature of the device. For example, a needle or probe may be inserted through rubber plug 9 of Fig. 1 to push armature head 31 against contacts 21 and 23 while an ohmmeter or like test instrument is applied to input leads 13 and 14. The same test can be accomplished by mechanical means especially provided for the purpose and built within the shielding of a missile when the radio frequency-proof firing device is incorporatedin a missile.

Although certain specific embodiments of the invention have been shown and described, it is obvious that many modifications thereof are possible; The invention, therefore, is not to be restricted except insofar as is necessitated by the prior art and by the spirit of the appended claims.

,What is claimed and desired to be secured by Letters Patent is:

l. A radio-proof electric initiator comprising a casing, a heat sensitive ignition composition housed in said casing, a pair of input lead wires entering said casing, a bridge wire disposed in heat exchange relation to said ignition compositioma coil supported by said casing and connected to said input lead wires, said coil having a designed overall impedance sufficient to prevent any induced R.F. currentfrom energizing the coil. to attract an armature associated therewith, a tap on said coil for shunting a portion of said coil when proper firing current is applied to the coil, a pair of spaced contacts one of which is connected to said tap of the coil, the second contact being connected to one end of said bridge wire, the other end of saidbridgewire being connected to one of said input lead wires, and an armature operative to bridge said contacts when said coil is energized by a proper firing current, exclusive of R.F. currents, ap-

plied to said input lead wires.

. when proper firing current is applied to the said input lead wires, and the wire size of that coil section which continues to draw firing current after attraction of said armature being of larger size thanthat of the said shunted section.

4. A radio-proof, electric initiator according to claim 2 wherein said coil is wound on a tubular insulating body open at one end, said tubular body seating a spring within its opening in biasing relation with said armature to urge the armature away from said pair of spaced contacts.

5. An electric initiator according to claim 2 wherein is additionally provided a closure plug for said cap shell through which said lead wires pass, a recess in the inner side of said closure plug to receive and limit the motion of said armature, said coil beingwound on a tubular body open at one end, and a spring seated in said body biasing said armature away from said pair of spaced contacts into said closure recess.

6. An electric initiator as described in claim 2 wherein said coil is wound on a tubular insulating body open at one end, said pair of spaced contacts being supported on said body at its open end, and a spring seated in said body biasing said armature away from said pair of spaced contacts. p

7. A radio-proof, electric initiator comprising a cap shell, a heat sensitive ignition composition housed in said cap shell, a bridge wire disposed in heat exchange relation to said ignition composition, an ignition plug adjacent said ignition composition, a plug sealing one end of said cap shell and spaced from said ignition plug, a pair of input lead wires entering into the cap shell through said cap shell sealing plug, a coil supported within the cap shell and connected to said input lead Wires, ,said coil being positioned between said ignition plug and said cap shell sealing plug and having a designed overallimpedance sufiicient to prevent any induced R.F. current from energizing the coil to attract rent is applied to the coil, a pair of spaced contacts one of which is connected to said tap of the coil, the second contact being connected to one end of said bridge wire, the other end of said bridge wire being connected to one of said input lead wires, an armature positioned Within said coil and having a head resting on said ignition plug, and means to bias said armature head away from said spaced contacts, said armature being operative to overcome said bias means and bridge said contacts when said coil is energized by a proper firing current, excluding R.F. currents, applied to said input lead wires.

8. An electric initiator according to claim 7 wherein said pair of spaced contacts are insulatedly supported on that end of said coil adjacent to said ignition plug, and said armature bias means comprises a resilient ring 'positioned between said coil and said armature head.

9. An electric initiator according to claim 7 wherein said pair of spaced contacts are insulatedly supported on that end of said coil adjacent to said ignition plug, and said armature bias means comprises a resilient plastic ring positioned between said coil and said armature head, said ring being apertured diametrically in its wall to permit sliding movement therethrough of the body of said armature.

10. The combination with an electric initiator having a bridge wire disposed in heat exchange relation to a heat sensitive ignition composition, of a radio-proof electric firing device comprising a pair of input lead wires, a coil wound with wire of two sizes connected to said input lead wires, that portion of said coil which is wound v with the smaller size wire being shunted by a tap when proper firing current is applied, said coil having a designed overall impedance sufiicient to prevent any induced R.F. current from energizing said coil and attracting an armature associated with said coil, a pair of spaced contacts one of which is connected to said tap of said coil, the second contact being connected to one terminal of said bridge wire, the other terminal of said bridge wire being connected to one of said input lead wires, and an armature operative to bridge said contacts when said coil is energized by a proper firing current selected from a range of low frequency AC. and DC. currents and applied to said input lead Wires.

References Cited in the file of this patent UNITED STATES PATENTS 2,331,058 Stick Oct. 5, 1943 2,818,020 Burklund Dec. 31, 1957 FOREIGN PATENTS 703,790 Germany Feb. 13, 194 1 

